The present invention relates to voltage or current sense amplifiers, and more particularly, to a sense amplifier that can sense the full wave of the alternating current (AC) in a discharge lamp.
A discharge lamp, such as a cold cathode fluorescent lamp (CCFL), has terminal voltage characteristics that vary depending upon the immediate history and the frequency of a stimulus (AC signal) applied to the lamp. Until the CCFL is xe2x80x9cstruckxe2x80x9d or ignited, the lamp will not conduct a current with an applied terminal voltage that is less than the strike voltage. Once an electrical arc is struck inside the CCFL, the terminal voltage may fall to a run voltage that is approximately ⅓ of the strike voltage over a relatively wide range of input currents. When the CCFL is driven by an AC signal at a relatively high frequency, the CCFL (once struck) will not extinguish on each cycle and will exhibit a positive resistance terminal characteristic.
Driving a CCFL with a relatively high frequency square-shaped AC signal will produce the maximum useful lifetime for the lamp. However, since the square shape of an AC signal may cause significant interference with other circuits in the vicinity of the circuitry driving the CCFL, the lamp is typically driven with an AC signal that has a less than optimal shape such as a sine-shaped AC signal.
Typically, the lamp is driven by an inverter, which converts a DC signal to an AC signal, filters the AC signal, and transforms the voltage to the higher voltages required by a CCFL. Examples of such inverters are shown in U.S. Pat. No. 6,114,814 to Shannon et al., assigned to the assignee of the present invention and herein incorporated by reference in its entirety. Also, the MPN1011, MP1015, and MP1018 products from Monolithic Power Systems, Inc. are exemplary of the type of inverter-used to drive a CCFL.
In order to most efficiently deliver power to the lamp, it is necessary to monitor the current delivered to the lamp. Therefore, a sense amplifier is used to monitor the lamp current.